The present invention generally relates to compound semiconductor crystal growing methods, and more particularly to a compound semiconductor growing method which controls adsorption of a very small amount of material gas on a substrate surface.
Recently, active research and development are made in respect of various kinds of semiconductor devices, and there are many kinds of semiconductor crystals used in such semiconductor devices. Compound semiconductor crystals and their mixed crystals are essential to the semiconductor devices having various kinds of functions. Accordingly, the growth speed, the crystal composition, the impurity concentration and the like are very important control factors when growing the compound semiconductor crystals.
Conventionally, when growing a crystal layer of a compound semiconductor using a chemical vapor deposition(CVD), the growth speed, the crystal composition and the impurity concentration are controlled by controlling the mole fraction of the material gas which is supplied to a reaction chamber. More particularly, the flow rate of the material gas is controlled by use of a mass flow controller.
However, the accuracy of the mass flow controller is extremely poor if the flow rate is only on the order of several sccm. As a result, there was a problem in that an accurate impurity concentration cannot be obtained if the doping is made with a very small amount of impurity.
In addition, because of the poor accuracy of the mass flow controller, there were problems in that the reproducibility of the process is poor and that it is impossible to arbitrarily control the growth speed, the crystal composition and the impurity concentration.
Accordingly, it is conceivable to use a diluted gas in order to suppress the undesirable effects caused by the poor accuracy of the mass flow controller. For example, when supplying 1 sccm of material gas, a mixed gas made up of 10 sccm of the material gas and 490 sccm of diluting gas such as hydrogen is first obtained, and 1/10 of this mixed gas is then supplied to the reaction chamber so as to substantially supply 1 sccm of the material gas to the reaction chamber. In this case, the mass flow controller can measure 10 sccm of the material gas with a relatively high accuracy. For this reason, even if the doping is to be made with a very small amount of impurity, it is possible to obtain a relatively accurate impurity concentration when compared to the conventional method.
But this conceivable method which uses the diluted gas must first generate the mixed gas and then supply a part of the mixed gas to the reaction chamber. Consequently, there are problems in that the control of the apparatus becomes complicated, the apparatus becomes large and the apparatus becomes expensive.
On the other hand, according to the conventional method and the conceivable method which uses the diluted gas, the impurity concentration is controlled by controlling the supply of the doping (material) gas. Thus, when forming a multi-layered structure having different impurity concentrations, it is necessary to once interrupt the growing process at the layer boundary and reset the amount of doping gas to be supplied. However, the doping gas in general has a strong toxic character, and the doping gas must be handled with extreme care. For this reason, as a safety measure, it is desirable to minimize the operations involving the doping gas, such as interrupting the supply of the doping gas and resetting the supply quantity of the doping gas.
Furthermore, if the doping is made by the conventional method to an extremely high impurity concentration, the composition of the doped layer slightly changes. But conventionally, there was no method of preventing such a change in composition caused by the doping to the high impurity concentration.